1. Field of the Invention
This invention relates to flatting agents, and more specifically to inorganic hydrogel flatting agents characterized by their high pore volumes, small particle sizes, and narrow particle size distributions.
2. Description of the Prior Art
It is known in the prior art that synthetic or natural particulate materials can be used as flatting agents in various applications such as industrial coatings, synthetic leather, plastics, printing, etc. Ideally, flatting agents should possess the following properties: high pore volume; narrow pore size distribution; appropriate particle size for the particular application; narrow particle size distribution; and maintenance of particle integrity, pore volume and particle size distribution during processing.
Because they possess most of the above properties, precipitated silicas, silica aerogels and xerogels are commonly used as flatting agents. The process for making silica aerogel or xerogel flatting agents is well known to those skilled in the art, and is represented in FIG. 1.
Under this process sodium silicate and sulfuric acid are mixed rapidly and continuously at low temperatures, low pH, and high concentrations to form a hydrosol. The hydrosol sets and undergoes a phase change to a gel-like structure known as a hydrogel. The hydrogel is broken into relatively small sections and washed to remove soluble salts and impurities. It is during this wash cycle that the pore structure of the washed hydrogel is developed. Following the wash cycle, the washed hydrogel is then dried and/or activated by thermal means to form silica gel which can then be ground or milled to a specific particle size and particle size distribution. In some cases, application of a surface treatment to provide lubricity or to improve suspension properties is also performed.
The final gel properties can be controlled by the rate and method of drying. For example, when the hydrogel is dried slowly, the pore structure collapses and results in a xerogel. Xerogels are characterized by a compressed structure, reduced pore volume and relatively high surface area. The pore volumes of flatting types of xerogels are typically around 1.1 ml/g. In contrast, when the liquid in the washed hydrogel is removed by rapid drying, by solvent extraction, azeotropes, or other similar means to reduce the surface tension of the liquid within the hydrogel pores, shrinkage is reduced, the original pore volume is substantially preserved, and an aerogel structure results. Aerogel flatting agents are therefore characterized by their higher pore volumes typically around 1.4 to 1.7 ml/g, and relatively lower surface areas.
There are certain disadvantages in using any previously existing form of silica flatting agent. As previously discussed, xerogels are characterized by their compressed structure and resultant reduced pore volume and thus exhibit reduced flatting efficiency. Aerogels, with their comparatively higher pore volume are among the most efficient flatting agents currently employed in the coatings industry. However, there are substantial capital equipment costs attributable to the water removal, drying and activation processes involved in the manufacturing of aerogel flatting agents. Precipitated silica flatting agents may have flatting efficiencies as high or higher than aerogels, but are inferior otherwise due to their friable nature in terms of maintaining suitable particle size during agitation.